The present invention relates to earth boring tools and, more particularly, to such earth boring tools as hole openers, raise drills, and big hole cutters.
Earth boring tools typically include cutters mounted to rotate on journals or mounting shafts. Some of these cutters define the diameter of the hole being bored. These diameter defining cutters are known as gage row cutters. Viewed in the vertical, the axis of the journal of each gage row cutter extends laterally from the vertical rotational axis of the tool and at an acute angle to a plane normal to this vertical axis. Gage row cutters are typically in the form of truncated cones and have steel teeth or tungsten carbide inserts on their conical faces for engaging and cutting the rock. The orientation of the cones presents only a point of the cutter to the full bore diameter at any instant of time. Rotation of the tool about the vertical axis rotates the cutters on formation rock and the cutters crush material in their path. The central portion of the hole may be drilled by a conventional rock bit, connected to the drilling apparatus, or the control portion of the hole may be a pilot bore previously made.
The outer cutters are known as gage row cutters because they determine the diameter of a hole being bored. Very little wear at the gage row can be tolerated because the hole diameter must be set within narrow tolerances. However, the rate of wear of the cutter at the gage row is comparatively great. This is so because the cutters rotate at a comparatively high rate of speed due to their extreme radial position from the axis of rotation of the tool. Also the gage row cutters do most of the boring work and are in a zone where cuttings accumulate and abrade their cutting surfaces. For protection against the harsh abrasive environment adjacent to the peripheral wall of the hole being drilled, a shirttail forms a wear surface just inside of the radial extreme of each cutter and at the end of a yoke leg that supports one end of the cutter. The shirttails cover the cutters except for that small section that sees full gage diameter.
Cutters typically are mounted on their journals by bearings and these bearings are protected by seals. Loss of the seals can cause loss of lubricant, or intrusion of abrasive material, or both, causing loss of the bearings. It is desirable to be able to inspect and replace, if necessary, the cutter's seals without renewing its bearings so that the life of the cutter can be extended.
The yoke legs of the rock drilling tool that support the outer radial end of the gage row cutters tend to wear rapidly on their outer radial surfaces, even when these surfaces are formed of wear resistant shirttails. With the shirttail integral with the legs, it is difficult to replace the leg, especially in the field. With the shirttail integral with the journal of the gage row cutter, a sound journal, entirely machined and heat treated, would have to be replaced with a worn shirttail. Excessive wear of the shirttail also exposes the bearing seal, causing rapid loss of the seal.
It is thus highly desirable to be able to remove shirttails without affecting cutter mounting legs or journals.